Donald, however, had five difficult holes left to play at the Merion Golf Club before his first round could be completed and Phil Mickelson stood an excellent chance of eventually becoming the leader when the rain-disrupted first round comes to an end Friday.
Tiger Woods, meanwhile, endured a struggling start and appeared to injure his left wrist on the opening hole. He twice grimaced in pain during the course of the round as he attempted to advance his ball from the thick rough, and when play was suspended he was six shots out of the lead in a tie for 51st place.
Two rain delays totaling more than 4 hours left the afternoon starters with no chance of completing the opening round. Play was scheduled to resume at 7:15 a.m. Friday and it appeared all but certain the second round would not be finished until Saturday morning.
Donald finished 13 holes Thursday in 4-under par, reaching that figure with birdies on each of his last three.
Mickelson played in the morning and was the leader in the clubhouse at 3-under 67, having started his round only a few hours after completing an overnight flight from his home in suburban San Diego. He returned to California earlier this week to attend his daughter's eighth-grade graduation ceremony.
Masters champion Adam Scott was also at 3-under with seven holes remaining while defending champion Webb Simpson, Mathew Goggin and Alastair Presnell were all at 2-under. Simpson had 10 holes yet to play. Both Goggin and Presnell had 12 remaining.
The only early starter other than Mickelson to break par at Merion on Thursday was Nicolas Colsaerts, who shot a 1-under 69. There were eight other players at 1-under still on the course when the action was halted, among them Lee Westwood through 13 holes.
Westwood had reached 3-under before suffering a double-bogey at the par-4 12th.
Woods, seeking his first major title in 5 years, three-putted the opening hole for a bogey and suffered three more bogeys on the front nine. He stood at 2-over when the horn sounded and will be facing a difficult par putt on the 11th green when he returns to the course on Friday.
There had been speculation that a week of rain would soften the relatively short Merion course to the point that low scores would be commonplace from the best golfers in the world. That did not happen in the first round.
Dustin Johnson, Bubba Watson, Steve Stricker and Justin Rose were among those who shot a 71. Sergio Garcia took 14 shots to play the fifth and sixth holes before rallying to shoot a 73 while former U.S. Open champ Lucas Glover and Jason Dufner both had a 74.
Rory McIlroy, playing in the same group with Woods and Scott, was even par through 11 holes.